1. Field of the Invention
The present invention relates generally to wireless systems, and particularly to cognitive radio systems.
2. Technical Background
According to some reports, internet traffic has roughly doubled every year since 1997. At the same time, the public is becoming increasingly dependent on mobile telephones for voice, voice messaging, email and Internet access services. There is now therefore a huge demand for both wireless telephone devices and access to wireless service provided by the various wireless carriers.
All wireless signals propagate over the air via one or more radio frequencies. In layperson's terms, therefore, the term “spectrum” refers to all of the radio frequencies available for the transmission of voice, video, audio and other kinds of data over the air. Note that radio operators cannot transmit radio signals over the same frequencies at the same time because they would interfere with each other. Because of the demand for wireless services mentioned above, one of the biggest issues facing wireless carriers is the scarcity of available “spectrum.”
It turns out that the Federal Communications Commission (FCC) has allocated various parts of the electromagnetic spectrum to various purposes such as AM radio, VHF television, cell phones, CB radio, and so on and so forth for large sums of money. For example, more than $19.0 billion was spent for space in the 700 MHz portion of the spectrum in a 2008 auction. At the same time, the explosive growth in wireless applications and devices mentioned above will only continue. Accordingly, the communications bandwidth is crowded and it is becoming increasingly difficult to find available spectrum for new services or for the expansion of existing services.
On the other hand, the FCC has recognized that depending on the location and the time of day, up to 70% of the licensed spectrum may be idle. In other words, after obtaining a license to use a block of the radio spectrum, many licensees only use their licensed block during certain times in the day; the remainder of the time the spectral blocks are idle. The FCC is therefore encouraging the development of cognitive radios and dynamic spectrum access (DSA) technology that would allow unlicensed users to use the idle spectrum without infringing the rights of the licensed users.
Cognitive radio is a term that refers to a wireless communication scheme wherein the wireless network or the wireless device can dynamically change its transmission and reception parameters without interfering with licensed incumbent users. Thus, cognitive radio promises to help solve the spectrum scarcity problem mentioned above. The fundamental means for implementing a cognitive radio framework or scheme is a software defined radio. As its name suggests, the software radio hardware is programmable such that transmission and reception parameters (frequency, power, modulation type, etc.) can be adjusted on the fly to provide dynamic access to available spectrum. Many cognitive radio schemes are configured to negotiate with other devices in the vicinity to gain access to the unused spectrum. One of the drawbacks associated with this approach is that the negotiation itself requires the use of overhead bandwidth and is thus inefficient. Moreover, the additional bandwidth required to accommodate the negotiation exacerbates spectral scarcity and is thus self-defeating.
What is needed therefore is a cognitive radio approach that dynamically determines which frequencies in the electromagnetic spectrum are idle without requiring any overhead. A cognitive radio is needed that can detect the portion of the spectrum that includes a transmitted message intended for the receiver. A cognitive radio is also needed that can dynamically map its transmissions into idle subcarrier blocks on a symbol-by-symbol basis.